This invention relates to optical spectrum analysis utilizing interference techniques, and more particularly to a common path interferometer for such purposes.
A grating or prism spectrometer disperses radiation in terms of frequency and in recorded form provides a spectrogram which is a presentation of frequency versus energy. On the other hand, an interferometer is entirely non-dispersive, and its output is an interferogram, which is a display of the distance traversed by a mirror versus energy. By means of an inverse Fourier transformation, the distance versus energy pattern of the interferogram is converted into the more meaningful pattern of the spectrogram in the form of frequency versus energy.
Fourier transform spectroscopy is capable of greater sensitivity and response time than the more conventional dispersive methods, and is usually implemented by means of a Michelson interferometer. A Michelson interferometer consists of a beam splitter which sends half of the radiation it receives to a moving mirror and the other half to a stationery mirror. Upon reflection from the mirrors, the two beams are recombined at the beam splitter, producing interference fringes and accordingly an output interferogram. The mirror in one arm of the interferometer must move perfectly parallel to itself and its position at any point with respect to the output interferogram must be known to a precision better than a wavelength of light in order to recover the spectrum. This places extremely stringent requirements on the mechanisms producing the motion and measuring its position. Also, the differential path length is dependent on the length of the two separate arms of the interferometer as well as the orientations of the beam splitter, the movable mirror, and the fixed mirror.
It would be highly desirable to cause the radiation to be analyzed which is broken up into two beams to traverse the same geometric path while optically still delaying one with respect to the other, and recording the pattern formed by the superposition of the beams. This is called a common path interferometer, and has been accomplished in one form by polarized light where one plane of polarization is delayed with respect to the other by a birefringent crystal. Scatter plate interferometers are also common path, but are highly wasteful of light and are generally used only for optical inspection purposes. Neither of these methods is practical for general purpose spectral analysis.